During the manufacture of webs of flat sheet materials, such as paper, plastic films, textiles and the like, the webs are monitored by devices, which may be referred to herein as detectors or sensors interchangeably, that feedback information used to control manufacturing. Manual or automated process control systems may use this information. Sensors used in process control systems need to accurately measure properties of a quickly moving, fluttering web, while operating in a high humidity, dirty, hot and/or wet environment. Such sensors usually mount on measurement platforms that scan the sensors in a cross machine direction (CD) as the process web moves relatively rapidly in a machine direction (MD).
Infrared spectroscopic sensors are common monitoring devices for such control systems. These sensors measure the absorption of infrared radiation at specific wavelength bands, indicating a specific property's presence and/or magnitude. Specific characteristics that the sensors may measure include properties such as water, polymers, coating minerals, cellulose and other components of a web. A common application is the measurement of the fraction of water by weight (percent moisture) in a moving paper web during manufacturing.
The infrared spectroscopic sensor measurements utilize the differential absorption of various wavelength bands in the near infrared region, generally 0.75 μm to 10.0 μm, by water and other components of the web. Process controllers compare measurements of the transmission and/or reflection of infrared energy at one or more reference wavelengths to measurements of the transmission and/or reflection at one or more absorption wavelengths. The reference wavelengths are selected for a relatively low absorption coefficient by as many of the components of the web as possible, and the absorption wavelengths are selected for a relatively high absorption coefficient. A number of different wavelength measurements may be used to determine and/or reject other interfering parameters, such as the mean optical path length through the web as a result of optical scattering.
The infrared radiation sensors, such as lead sulfide (PbS), lead selenide (PbSe) or Indium Gallium Arsenide (InGaAs) sensors, generally measure infrared energy in several spectral bands, making all measurements simultaneously with the measurements being representative of the same area of the moving process web. Simultaneous measurement generally requires multiple, spatially separated sensors, each of which detects infrared energy at one of the spectral bands of interest. Since the properties of the web that affect the various infrared wavelengths can vary over short distances on the web, any differences in the web areas presented to the sensors may result in measurement error. Simply placing the individual sensors proximate to each other is generally inadequate to meet accuracy requirements. The signals from these sensors may be mathematically combined to develop measurements of interest.
Indium Gallium Arsenide (InGaAs) sensors are generally preferred since lead salt sensors are more sensitive to temperature and thus require more frequent normalization to correct errors produced by temperature drift and sensor dark current. Almost all currently available web property measuring systems normalize sensor signals by using continuous chopping devices such as filter wheels, tuning forks, rotating blades, shutters or the like. Traditional continuous chopping reduces the measurement signal. At best, a beam having a sinusoidal chop has half the average energy of a beam that is not chopped.